1. Field of the Invention
This invention relates to a hydraulic strut for a motor vehicle for providing a movement guiding and dampening function. Such a strut can generally have a cylinder, with a piston disposed in the cylinder to divide the cylinder into at least first and second chamber portions, with a piston rod extending from the piston out of the cylinder. The cylinder can be sealed at both ends, and can preferably be filled with hydraulic fluid. The piston can preferably define a flow connection between the two fluid chambers, and this flow connection can typically have some sort of valve arrangement therein for controlling flow of fluid between the first and second chamber portions. Such a piston-cylinder arrangement is typically well known for items such as shock absorbers for suspension systems of a motor vehicle. By means of attachment devices, the cylinder can then be attached to a first object, i.e., a first portion of a motor vehicle and the extending end of the piston rod can be attached to a second object, i.e. a second portion of a motor vehicle.
The strut can thereby guide movement between the first and second objects while providing a fluid dampening function. In addition to providing a movement guiding function, such a strut can also be used for essentially maintaining a relative positional relationship between the two objects to which it is attached, and for this function, can preferably be designed for providing a position locking device for positioning objects which are movable with respect to one another. Such a positioning device can have a hollow cylinder which can be connected to one of the objects and a piston which can be displaced in the hollow cylinder, which piston can be connected to the other object by means of a piston rod which runs out of the hollow cylinder. The piston can divide the externally-closed hollow cylinder into two working chambers, which working chambers can have variable volumes containing a working fluid and/or a gas, whereby the flow connection between the two working chambers is possible at least partly by means of an exchange device in the piston.
2. Background Information
One such strut device is disclosed by German Patent Application No. 14 59 182 C2.
Known devices have a cylinder which is closed on both ends, with a piston rod which extends through one end of the cylinder in a sealed manner. Inside the cavity of the cylinder, the piston rod is connected to a piston, which piston divides the cylinder into two working chambers. The two working chambers are connected to one another by means of two flow passages which run inside the piston. For each direction of movement there is a non-return valve which can open only in one flow direction. Each non-return valve comprises a valve body which is prestressed by a spring in the blocking position against an inflow opening, and which blocks the opening when the piston is stationary. When the piston rod is moved relative to the cylinder, an overpressure builds up in one of the working chambers. This overpressure acts on the corresponding non-return valve. When the piston rod begins to move, this pressure initially acts only on a small area of the surface of the valve body, which area is defined by the cross section of the inflow opening corresponding to the valve body. A significant overpressure in the working chamber is therefore required to lift the valve body from the sealing position. After the flow between the two working chambers begins, pressure is exerted on a larger area of the surface of the valve body.
This known solution makes it possible to hold a door in a relatively stable manner in any desired position between its open position and its closed position. A relatively large force must be applied to start to move a door or similar object which has been arrested in a desired position. The door therefore essentially remains in its position even in the event of an unintentional impact. On the other hand, a relatively small force is required to open or close the door, once the door has been set in motion. This action, which is itself advantageous, is unfortunately achieved only at the expense of an increase in the cost and complexity of the design and construction, which has so far kept this principle from being adopted in widespread applications. The added cost and complexity of design and construction results from the fact that there must be two flow paths between the two working chambers. These two flow paths require a number of holes which form the flow paths, some of which are axial holes which must be located eccentrically in the piston.